This is low key funny and adorable to me,
but in all seriousness, I don't know what they talk about you, but my closest friends and I do talk about my romantic life in the sense if I'm dating someone, we do talk about said bf a lot. That's usually because they'd know when I develop feelings for the person and are there through the journey and it's a nice thing to talk about.
Sometimes its the idea of showing off your relationship, sometimes its looking for their approval of sorts, and sometimes its just cause its interesting or you want them to have your back on some issue.
It can be annoying having them talk about you behind your back always, talk it out with your gf maybe
Don't you talk about your gf to your guy friends too?